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Estimates now used by the Environmental Protection Agency (EPA) and independent researchers to determine greenhouse gas (GHG) emissions from upstream shale gas production likely are "significantly overstated" because they are based on assumptions that don't reflect current industry practices, according to IHS Cambridge Energy Research Associates (IHS CERA).

"Methane emissions have become a very important and controversial issue given their potency as a greenhouse gas," said Barcella, director of North American natural gas. "Unfortunately, such emissions are not being measured. Estimates are being used that are not supported by data, do not reflect current industry practice and would be unreliable to use as a base for decision-making."

In one example, the authors cited EPA's 2010 revised estimates of methane emissions during well completion, which is the period after the well has been drilled but before it is placed into production. Current EPA methods to estimate methane emitted during this phase were based on a "small sample of wells and primarily measured methane that was captured rather than released into the atmosphere," the IHS CERA team said.

According to the report, EPA's estimates were based on two workshop presentations describing methane captured during "green completions," which are operations designed to capture as much methane as possible. EPA apparently assumed that similar levels of methane were produced at every other well in the United States and that those emissions went completely uncaptured. However, those assumptions don't conform to current industry practices, Barcella and her colleagues noted.

"The assumption that all methane recovered from these sample wells would otherwise have been flared or vented is questionable at best, given that common industry practice is to capture gas for sale as soon as it is technically feasible," said Rajan. "Gas that cannot be sold is generally flared rather than vented for safety reasons. If the methane emissions at wells were as high as some methodologies assume, you would have extremely hazardous conditions at the well site that neither regulators nor industry would permit."

Another " key mischaracterization" in EPA estimates and those in recent reports "is the assumption that wells in flowback contain methane in quantities equal to their post-completion daily production," said the authors. "This assumption results in a significant overestimation of methane emissions." Flowback is the phase of production when fluids injected into the well flow back out ahead of the tapped gas.

IHS CERA's report disputes findings made earlier this year by Cornell University, which claimed that methane emissions of flowback gas when wells were completed were high enough to increase the GHG footprint of shale and tight gas to levels that exceed those of coal (see Shale Daily, April 13). However, a follow-up study by gas industry-supported American Clean Skies Foundation challenged the Cornell claims (see Shale Daily, April 21). Separately energy consultant Wood Mackenzie also found that Cornell researchers had significantly overestimated the fugitive methane emissions from unconventional gas wells (see Shale Daily, May 11).

Basically, said the IHS CERA directors, data on unconventional gas well GHG emissions is currently lacking because the emissions haven't been adequately measured to produce estimates "with any degree of certainty."

New air emissions standards proposed by EPA last month may prove to be a more accurate method of collecting GHG data, according to the authors. In late July EPA proposed new source performance standards under the Clean Air Act that would regulate air emissions during the completion phase of hydraulically fractured gas wells (see Shale Daily, July 29). The proposed standards don't directly regulate emissions of methane or other GHGs but instead focus on sulfur dioxide and volatile organic compound emissions. However, the measures that reduce these pollutants' emissions have the additional benefit of reducing methane emissions as well, they noted.

"The proposed standards have the potential to codify good operating practice in the gas drilling industry. The data collection requirement could also provide much more reliable data on methane emissions from gas well completions, a potential benefit to all who seek to better understand GHG emissions from the industry."

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